Cardiomyocytes in the adult mammal exhibit little if any capacity to undergo cell division. Consequently cardiomyocyte loss due to injury or disease is irreversible. Identification of the gene products which regulate cardiomyocyte proliferation and terminal differentiation might provide useful molecular targets with which to induce therapeutic myocardial growth in the adult heart. We have used cardiomyocyte cell lines derived from transgenic mouse tumors to identify 3 proteins (p193, p107 and p380) which bind either directly or indirectly to the SV40 Large T-Antigen (T-Ag) oncoprotein. In addition, we have cloned the mouse cDNA and genes from the Tuberous Sclerosis Complex (TSC) 1 and 2 loci: TSC is an autosomal dominant familial cancer which affects a variety of organs including the heart. We hypothesize that these proteins participate in the regulation of cardiomyocyte proliferation and/or terminal differentiation. In support of this, experiments performed during the current funding period have shown that p193 exhibits tumor suppressor activity in NIH-3T3 cells, and that p107 and TSC2 impact upon cardiomyocyte terminal differentiation and proliferation, respectively. In this competitive renewal application, we propose to further test our hypotheses with additional gain and loss of function models. Four Specific Aims are proposed. In Aim number 1, we will generate gain and loss of function transgenic mice to ascertain the role of p193 in normal and pathologic cardiac development, as well as identify the cellular proteins which interact with p193. In Aim 2, we will establish the molecular mechanism by which p107 expression renders the hearts of transgenic mice resistant to isoproterenol-induced hypertrophy, as well as determine if these animals are resistant to other hypertrophic stimuli. In Aim 3 we will isolate and clone p380, a novel myocardial p53 binding protein, and will generate gain and/or loss of function transgenic models to directly test its role in cardiac growth and development. Finally, in Aim 4 we will generate TSC1 deficient ES cells to test the role of this gene product in the regulation of cardiomyocyte proliferation in vitro and in vivo. We will also generate transgenic mice which express TSC2 dominant negative mutants in the myocardium. If the putative regulatory roles for these proteins are confirmed, they may serve as intracellular targets for therapeutic myocardial growth in the adult heart.